Cobaloximes, bis-dimethylglyoximato complexes of cobalt, have been used as model compounds in the study of biochemistry of vitamin B.sub.12. Cobaloximes and cobalamins (vitamin B.sub.12 and its derivatives), exhibit very similar chemical and photochemical behavior by virtue of the nearly identical coordinating strengths of the macrocyclic dimethylglyoxime and corrin (vitamin B.sub.12) ligand systems. Five coordinate cobaloximes and cobalamins, where cobalt is in the Co(II) oxidation state, have a high affinity for radicals and react via adduct formation, which involves the generation of a carbon-cobalt bond and .beta.-hydrogen abstraction, which gives an olefin and a cobalt (Ill) hydride, at rates which are essentially diffusion controlled. Alkyl derivatives of cobaloximes and cobalamins can be synthesized and exhibit photochemical behavior which includes the ability to generate radicals reversibly by means of photolytic homolysis of the carbon-cobalt bond.
Steric factors play an important role in the chemistry of cobaloximes and cobalamins, due to the considerable bulk of their macrocyclic ligand systems. The relative magnitudes of these steric factors govern both the stability of the six coordinate cobalt (Ill) alkyl derivatives and the reactivity of the corresponding five coordinate complexes.
One of the earliest applications of organocobalt chemistry to the field of polymer synthesis was in the area of catalytic chain transfer. The concept of catalytic chain transfer, first introduced by Enikolopyan et at., in 1981, involves the transfer of a hydrogen atom from a propagating polymeric radical to a monomer via interaction with an organocobalt catalyst, which is subsequently regenerated. The mechanism of the catalytic chain transfer reaction consists of the abstraction of a .beta.-hydrogen from the propagating radical by the catalyst to give a cobalt hydride and a polymer terminated by a double bond. The cobalt hydride subsequently reinitiates polymerization by transferring the hydrogen atom to a monomer molecule. The transfer reaction, which proceeds at very rapid rates (essentially diffusion controlled), results in polymer molecules terminated exclusively by double bonds. Although initial investigations of catalytic chain transfer involved the use of cobalt porphyrin catalysts, cobaloximes have since been demonstrated to be more efficient catalytic chain transfer agents, while possessing the added advantages of lower cost and better solubility than the porphyrins. Cobaloximes are however, sensitive to oxygen, requiring them to be handled under an inert atomsphere. The prior art has taught however, that the application of catalytic chain transfer is limited mainly to the polymerization of methacrylate esters and styrene.
Since the work of Enikolopyan, Smimov, Ponomarev, and I. M. Belgovskii, J. Polymer Sci., Polymer Chem. Ed., 19, 879 (1981), had demonstrated that the polymerization of acrylate esters does not result in catalytic chain transfer, but instead results in the formation of stable adducts between the propagating polymeric radical and the cobalt catalyst, it was thought that it would be possible that the alkyl, alkylaryl, arylalkyl and aryl cobaloximes photoinitiated polymerization of these monomers would give polymers with well-defined structures containing a controllable head group and terminated quantitatively by cobaloxime. What was not anticipated was that the termination of the polymer by cobaloxime could be reversible, thus resulting in a chain end that could undergo reversible photolytic homolysis to regenerate the active radical center and subsequently reinitiate polymerization. The net result of the characterization of this reversible termination behavior, was the development of a new living polymerization system, which proceeds via a photoinitiated free radical mechanism.